In recent years, for gasoline engines that serve driving systems for cars, etc., the introduction of gasoline engines of the fuel-direct-injection type that directly inject fuel into a combustion cylinder has been studied in order to improve the fuel economy and output.
In such a gasoline engine of the fuel-direct-injection type, during a constant-speed operation, it is preferable to operate the engine at a lean burn mode that provides an excessive amount of air-intake in relation to the fuel, rather than to operate it at a stoichiometrical air-fuel ratio that is an air-fuel ratio capable of purifying exhaust gas effectively, in order to improve the fuel economy.
In this gasoline engine of the fuel-direct-injection type, the temperature of the exhaust gas, measured at a position in which catalyst for purifying the exhaust gas is applied, is as low as 200 to 350° C., as compared with the temperature of the exhaust gas of a conventional gasoline engine that is operating at the same air-fuel ratio as the gasoline engine of the fuel-direct-injection type, that is, at the lean burn mode.
Moreover, in the gasoline engine of the fuel-direct-injection type, during an accelerating operation and a high-load operation such as operations upon passing and slope-climbing, it is preferable to operate the engine at an air-fuel ratio in the vicinity of the stoichiometrical air-fuel ratio in order to maintain a safe operation at high torques; and in this case, the exhaust gas temperature is in the range of 300 to 800° C. at the catalyst-applying position.
Conventionally, various researches have been carried out on catalysts for removing nitrogen oxides (hereinafter, referred to as NOx) from exhaust gas from an internal combustion engine under an atmosphere of excessive oxygen, that is, under an exhaust-gas atmosphere when it is driven at the lean burn mode.
For example, in the case when NOx is removed from exhaust gas from a stationary Diesel engine for a boiler, etc. among the internal-combustion engines, a method for adding reducing agents, such as ammonia, hydrogen or CO, to the exhaust gas so as to remove NOx by using the reducing agents is commonly used.
However, this method requires a device for separately loading the reducing agents and a special device for recovering and processing unreacted agents, thereby making the device for purifying exhaust gas complex and bulky.
Recently, in order to solve the above-mentioned problems, various methods for removing NOx from exhaust gas under an atmosphere of excessive oxygen have been proposed.
However, it has proven difficult for each of these methods to positively remove NOx from exhaust gas under all operation conditions because the exhaust gas has greatly changing concentrations of NOx, CO, HC and Oxygen as well as greatly vacillating temperatures depending on respective operation conditions, such as exhaust gas from a gasoline engine of the fuel-direct-injection type.
In other words, a method has been disclosed (see Japanese Laid-Open Patent Publication No. 125250/1985 (Tokukaisho 60-125250) and the specification of the U.S. Pat. No. 4,297,328) in which an NOx-decomposing catalyst consisting of crystalline aluminosilicate containing copper irons is used as the catalyst for removing NOx under an atmosphere of excessive oxygen.
The above-mentioned Laid-Open Patent Publication, however, merely discloses that nitrogen monoxide (NO) can be decomposed into nitrogen (N2) and oxygen (O2), and it is difficult for the above-mentioned method to remove NOx effectively under actual exhaust gas conditions.
Moreover, Japanese Laid-Open Patent Publication No. 100919/1988 (Tokukaisho 63-100919) discloses a method wherein exhaust gas is processed using a copper-containing catalyst under an atmosphere of oxygen in the presence of hydrocarbon (hereinafter, referred to as HC), that is, under an atmosphere of excessive oxygen, so that the reaction between NOx and HC preferentially progresses and so that NOx is effectively removed. This method is reported to be able to use either HC contained in the exhaust gas, or HC that is externally applied on demand.
The above-mentioned method includes a specific embodiment wherein exhaust gas is first allowed to contact the copper-containing catalyst so as to remove NOx and then contact an oxidizing catalyst so as to remove HC, carbon monoxide (hereinafter, referred to as CO), etc.
However, this method requires high optimal processing temperatures at which NOx can be sufficiently removed, and the removing effect is reduced at low temperatures; thus, the resulting problem is that this method is not sufficiently applied to removal of NOx from exhaust gas at the lean burn mode in a gasoline engine of the fuel-direct-injection type.
Furthermore, the catalyst in the above-mentioned method is inferior in heat resistance, and also has the problem of fast deterioration of the catalyst activity with time since the decomposing capability of NOx is irreversibly lowered in the case when it is exposed to high-temperature exhaust gas as in the case of the operation of a gasoline engine of the fuel-direct-injection type in the vicinity of the stoichiometrical air-fuel ratio.
In order to solve such problems, Japanese Laid-Open Patent Publication No. 171625/1989 (Tokukaihei 1-171625) discloses a method wherein: a copper-containing catalyst and an oxidizing catalyst or three way catalyst are placed parallel to each other in the flowing passage of exhaust gas, and when the exhaust-gas temperature becomes high, the gas is bypassed to the oxidizing-catalyst side or the three way catalyst side. However, the arrangement of the above-mentioned Laid-Open Patent Publication has the disadvantage of requiring a complicated purifying device for exhaust gas, resulting in a problem of high costs.
Moreover, the catalysts for purifying NOx, disclosed in the above-mentioned three Laid-Open Patent Publications, are inferior in the capability of purifying HC, CO and NOx in exhaust gas at the vicinity of the stoichiometrical air-fuel ratio, as compared with the three way catalyst.
Furthermore, International Patent Application WO 93/08383 discloses a catalyst which oxidizes and absorbs NOx from exhaust gas under an oxidizing atmosphere and discharges NOx to the exhaust gas under a reducing atmosphere, and a NOx-purifying method by the use of the catalyst.
However, in order to oxidize and absorb NOx, the catalyst of the above-mentioned Patent Application requires that nitrogen monoxide be preliminarily oxidized into nitrogen dioxide, and since the oxidization of nitrogen monoxide is difficult to carry out at low temperatures, it is difficult to apply this method to the exhaust-gas purifying of gasoline engines of the fuel-direct-injection type.
It is an objective of the present invention to provide a simple method that makes it possible to positively eliminate exhaust gas for a long time under all operation conditions, even though, like exhaust gas from a gasoline engine of the fuel-direction-injection type, the exhaust gas has noxious ingredients whose compositions vary depending on the operation conditions and whose temperature changes greatly depending thereon.